1. Field of the Invention
The present invention relates to gate valves and more particularly relates togate valves of the type designed for operation with high temperature, high pressure corrosive fluids such as those generated in coal gasification processes, uranium enrichment, and the like. Specifically, the invention is directed to gate valves of the block and bleed type which are used for applications requiring a minimum of leakage. In such blocking valves both upstream and downstream ports are sealed to permit evacuation or purging of the valve body at greater than line pressure to prevent through leakage of toxic media.
2. Description of the Prior Art
Over the years, a wide variety of gate valves and slide valves of various types have been developed. U.S. Pat. No. 380,037 discloses a valve for steam engines wherein valve members are supported on curved bows and are pressurized to expand against the opposing valve seats. U.S. Pat. No. 1,783,329 discloses a gate valve wherein opposite valve members are biased outwardly against the seats by a pair of coil springs. U.S. Pat. No. 2,541,968 discloses a gate valve wherein sloped wedging surfaces are provided for moving a pair of valve members outwardly against facing annular valve seats. U.S. Pat. No. 2,778,600 discloses a gate valve wherein leaf springs are provided for biasing a pair of valve gates to slide in a longitudinal direction with respect to each other and wedging forces are provided to cause the valve gate to move laterally apart for seating. U.S. Pat. No. 2,826,391 discloses a wedge valve wherein valve elements are wedged outwardly apart against valve seats. Another wedge valve is shown in U.S. Pat. No. 2,861,770. U.S. Pat. No. 3,478,771 discloses a sectional body gate valve with a valve seat scraper device having springs biasing the valve members apart from one another. U.S. Pat. No. 3,815,868 discloses a double disc gate valve wherein rotatable valve discs are used and are wedged apart for seating.
In these prior art arrangements the sliding or rotating joints which are provided to accomplish the desired movements of the gate valve are not suitable for use in high pressure, high temperature corrosive fluid applications such as coal gasification, uranium enrichment, or the like, because with such sliding or rotating joints there will be a buildup of material at the joint which binds the joint and jams the valve so that it does not operate properly. Also, parts which rub together can corrode and cause the valve to stick or not retract fully prior to lifting.